The table view separator is that thin line drawn by default between table view cells. You control how far it extends to the leading and trailing edges with the seperatorInset property. Since iOS 7 this has been an inset based on the table cell margins. In iOS 11 Apple changed that to inset from the cell edges by default. That makes it easier to extend to the full table width but if you have been using an inset you may want to check how it now looks on iOS 11.